The dihydric phenols have achieved significant success in their commercial applications. Dihydric phenols are useful in the commercial manufacture of various polymers including the polyarylates, polyamides, epoxides, polyetherimides, polysulfones, and the polycarbonates. Significant attention has been directed to the commercial preparation of the dihydric phenols. For many years it has been well known that the acid catalyzed reaction of phenol with specific aldehydes or ketones could prepare the 4,4'-dihydric phenol with specific groups derived from the aldehyde or ketone connecting the two phenolic rings. In particular when the phenol is reacted with acetone, the dihydric phenol 4,4'(dihydroxyphenyl)propane-2, hereafter referred to as bisphenol-A, is formed. This compound has particular utility in the synthesis and preparation of polycarbonates, polyarylates, and copolyestercarbonates as well as epoxies. In order to manufacture certain polymers, in particular the polycarbonates, the bisphenol-A must have a high level of freedom from impurities. In particular, this freedom from impurities must manifest itself as a freedom from impurities imparting an undesirable color either to the bisphenol-A or the products manufactured therefrom.
Additionally, since the process to manufacture bisphenol-A contributes substantially to the final cost of products manufactured therefrom, the process should be particularly efficient. Consequently much attention has been directed to the recovery of bisphenol-A after its preparation. Because of the economic factors involved, recovery of bisphenol-A from every process stream where it occurs can be significant in terms of increasing the yield of bisphenol-A and also reducing its cost. Thus various side- or purge-streams have been evaluated for their potential in terms of recovery of bisphenol-A.
In the down stream processing of the composition resulting from the acid catalyzed condensation reaction, a loss in the quantity of the desired dihydric phenol and a reduction in the quality of the color of the dihydric phenol composition has been observed when mixtures of the desired dihydric phenol, phenol, and isomers of the desired dihydric phenol are separated in conventional distillation trains to recover the desired dihydric phenol. Generally, without an additive, portions of these materials readily degrade to a very dark color and undergo substantial chemical transformation which is viewed in the context of its ultimate application as a degradation in quality. When this occurs, significant reductions in the quantity of recoverable desired dihydric phenol are observed.
It has now been found that this degradation of the desired dihydric phenol, usually bisphenol-A, as well as the formation of undesired colored impurities can be substantially inhibited by the addition of hypophosphorous acid, H3PO.sub.2, to the composition comprising dihydric phenol, phenol, and isomers of dihydric phenol prior to or concurrent with the distillation step.